1. Field of the Invention
The invention relates to operating a positioner with a first input for an input variable W for prescribing a desired value, a second input for a controlled variable X, an output for a manipulated variable Y, and an actuator that generates a first output signal depending on the input variable and the controlled variable. This signal is used for forming the manipulated variable.
2. Description of the Related Art
Positioners are used in a variety of technical processes to control the position of an actuating device in a lower-level control loop. For this purpose, an input variable is generated at the first input of the positioner by an actuator of a higher-level control loop. The position of the actuating device represents the controlled variable X which the positioner is to monitor. The positioner compares this controlled variable X continuously to the input variable W at its first input and adapts, in the sense of matching, a manipulated variable Y that is generated at its output to the desired value prescribed by the input quantity W. The actuating device thus influences a physical quantity which the controlled variable of the higher-level control loop represents and which is usually supposed to be maintained at the desired value.
The building of control loops from several higher-level/superimposed control loops makes possible rapid and stable control processes which permit good control particularly when disturbances are present. These types of control loops are described in the literature under the heading xe2x80x9cCascade Controlxe2x80x9d. The type of technical process itself that is to be controlled is generally not dependent on the positioner used, since the signals for the input variable W, controlled variable X and manipulated variable Y are usually standardized unified signals. However, the characteristics of the process are taken into account by the type of positioner and its parameters. For example, a distinction is made between non-continuous and continuous controllers. Within the continuous controllers, a distinction is made between P-controllers, PI-controllers, PD-controllers, and PID-controllers (Proportional, Integral, Differential).
The behavior of the positioner when errors occur is particularly important to the reliability of the technical process. For instance, a typical error might be the severing of signal lines for the input variable. If this happens, the widely used dual-line positioners lack the energy to operate, and so they do not generate a corrective signal at the output. If the actuating device is an actuating device with a pneumatic drive, a spring memory that is integrated in the drive typically forces the final control element into a safety position when the corrective signal fails to occur.
It is the object of the invention to further develop the positioner and method for using this positioner to make reliable operation possible and to provide a higher degree of safety of a process even for abnormal states of a signal that represents the controlled variable.
The object relating to the control method is realized by a method for operating a positioner having a first input for an input quantity for prescribing a desired value, a second input for a controlled variable, an output for a manipulated variable signal, and an actuator, comprising the steps of generating, by the actuator, a first output signal based on the input variable and the controlled variable, forming the manipulated variable using the first output signal, continuously monitoring a signal representing the controlled variable by an error detection device, detecting an abnormal status of the controlled variable signal by the error detection device, generating an error signal by the error detection device in response to the detection of an abnormal status, and then providing the error signal to a changeover unit, deactivating the actuator with respect to a signal representing the manipulated variable signal when the error signal is present and preventing a signal representing the controlled variable from influencing the manipulated variable, activating a controller by the changeover unit when the error signal is present, and generating, by the activated controller, a second output signal allocated to the input variable by the controller for forming the manipulated variable.
The object relating to the positioner is realized by a positioner device comprising a first input for an input quantity for prescribing a desired value, a second input for a controlled variable, an output for a manipulated variable, an actuator, which determines the manipulated variable depending on the input variable and the controlled variable, an error detection device, which is connected to the second input for the controlled variable, a changeover unit, upon which the error detection device acts by providing an error signal, and a controller, the error detection device switching the changeover unit when an error of a signal representing the controlled quantity is detected, so that the controller and not the actuator, determine the manipulated variable.
The invention is thus based on the surprising recognition that by monitoring the signal that represents the controlled variable X of the positioner and switching the positioner over from the actuator to a controller in case of an abnormal status of the signal representing the controlled variable X, it is guaranteed that, though control is generally degraded, the operating mode is continuously safe since the higher-level control remains active.
This continuously safe operating mode can be realized by way of an error detection device that processes the signal representing the controlled variable, this device acting on a changeover unit for an abnormal status of this signal such that the changeover unit deactivates the actuator and activates a controller instead. In this process, the actuator is deactivated with respect to the signal of the manipulated variable of the positioner so that neither the signals of the controlled variable X nor modifications of it influence the manipulated variable.